The present invention relates to an SHF output power and channel frequency switching apparatus in FPU system.
FIG. 1 is a general block diagram of a transmission HF section of an FPU system. In FIG. 1, to the IF input end is applied an intermediate frequency (IF) signal FM-modulated with a video signal from a control section of the FPU system. This intermediate frequency signal, which is an FM signal modulated on a carrier of 130 MHz, is first amplified by an IF amplifier 1 and fed to a frequency converter 2. This frequency converter 2 makes the modulated intermediate-frequency signal of 130 MHz mixed with a signal of 1370 MHz which is generated from a local oscillator 3, thereby producing a second IF signal of 1500 MHz. It also makes this second IF signal mixed with a local oscillation output from an SHF synthesizer 4, thus producing an SHF wave. This SHF wave signal is amplified up to a rated output by an SHF power amplifier 5 and fed as an SHF output to an antenna. A bias control circuit 6 shown is a circuit for changing the output power of the SHF power amplifier 5.
When the conventional FPU system is used for repeating in a close range of 1 km or below, and when the transmission power output is, for example, about 1 W, the normal wave of about -25 dBm and reflected wave of about -60 dBm can be received as shown in FIG. 3, that is, there is the so-called multipath effect which often causes troubles in the antenna directivity adjustment for the microwave. In order to counteract this effect, it was needed to increase the diameters of the parabola antennas, sharpen the directivity and direct it upward or to skillfully adjust the orientations of the antennas while detailed transmitting and receiving conditions are being sent between the transmitting and receiving sides.
If the transmission power output is reduced to, for example, 2 mW, the reflected wave level and normal wave level at the receiving side are about -90 dBm and -50 dBm, respectively as shown in FIG. 4. Since the minimum level which can be received is about -70 dBm, there is no multipath reception, that is, only the normal wave can be received.
The bias control circuit 6 is designed to change the transmission power output in the FPU system as the transmitter side so that only the normal SHF wave can be received without the multipath effect. Also, in the bias control circuit 6, since the frequency of the SHF wave to be transmitted is necessary to be selected and fixed in association with the receiving station, and since the SHF output power should be independent of the fixed frequency, a bias control signal is generated to prevent the SHF output power from being changed with the change of the frequency.
An example of the conventional circuit for changing the SHF output power will be described with reference to FIG. 2. FIG. 2 shows a specific example of the SHF power amplifier 5 and bias control circuit 6 illustrated in FIG. 1. In the prior art, the SHF output power can be switched between two values of, for example, 5 W and 1 W. To the bias control circuit 6 are supplied a 5 W/1 W switching command signal and a 7 G/10 G switching command signal which is produced from a control system circuit on the basis of a discrimination signal corresponding to one of the 7-GHz band channel frequency and 10-GHz band channel frequency which is selected in a ganged relation with the change of the oscillation frequency of the SHF synthesizer 4.
Switches S1 and S2 in FIG. 2 are operated by the 5 W/1 W switching command signal, and switches S3 through S6 by the 7 G/10 G switching command signal. Variable resistors VR1 through VR4 are adjusted to produce voltages corresponding to the combination of the power switching signal and the frequency switching signal and fed as gate control voltages to the gates of transistors FET 1 and FET 2 of the SHF power amplifier so that a certain power can be produced at each frequency. Variable resistors VR5 through VR8 are also adjusted to produce voltages as drain control voltages the drains of the transistors FET 1 and FET 2. The SHF power amplifier 5 is composed of several stages of FETs, and is changed in its amplification degree by the gate control voltages and drain control voltages of the FET 1 and FET 2 which are fed from the bias control circuit 6.
In the prior art two-position switching is made between 5 W and 1 W. It is possible to make multi-position switching for SHF power. In this case, a proper power level can be selected by a multi-position switching for SHF power in accordance with the repeating distance, for example, close-range repeating or long-range repeating, thereby removing the multipath effect.
FIG. 5 shows an example of the multi-position switching of SHF power which was tentatively thought of by the inventor (not prior art), or a circuit for making four-position switching of 5 W, 1 W, 0.1 W and 2 mW. In this example, it is necessary to stepwise change the drain control voltage and gate control voltage of the SHF power amplifier in eight stages as, for example, shown in FIG.6. For changing the control voltages, voltage dividing circuits can be used which are formed of combinations of fixed resistors. However, considering that the power supply voltages are not always constant, variable resistors are generally used, and thus variable resistors VR1 through VR 16 are necessary as shown in FIG. 5. Therefore, each time the number of switching positions is increased by one, four variable resistors are necessary to be added, thus increasing the circuit package space and adjusting time.
Simply improving the conventional circuits for SHF power multi-position switching will result in the increase in the circuit scale. Therefore, the system cannot be small-sized, and the adjusting time becomes long because the number of variable resistors increases.